But veterinarians know less about another potentially toxic substance that could be lurking in broodmares' feed: a mycotoxin called zearalenone. Fortunately, researchers are working to better understand the effects of this substance.

At the 2013 Society for Theriogenology Conference, held Aug. 7-10 in Louisville, Ky., Heath King, DVM, Dipl. ACT, an assistant clinical professor at the Mississippi State University College of Veterinary Medicine, presented a poster chronicling recent research on zearalenone's effects on mares' reproductive performance.

Mycotoxins are harmful secondary compounds produced by molds that are found in the soil and vegetable matter including grains, forages, and feed. They can form in the field both before and during harvest and can continue to form under suboptimal storage conditions after harvest. In horses these toxins can cause a wide range of clinical signs, including respiratory, gastrointestinal, neurologic, and reproductive problems—even death.

Researchers know that zearalenone—primarily produced by the mold Fusarium graminearum and commonly found in barley, oats, wheat, corn, silage, rice sorghum, and some forages—has estrogenic effects (causes estrus) in swine and causes reproductive problems in cattle. However, its effects on equine reproduction remain relatively unknown, King said. So he and colleagues set out to evaluate the reproductive efficacy of healthy mares consuming two different zearalenone concentrations daily.

The team separated 21 mature, reproductively healthy mares into three groups of seven. One group served as a control and consumed one-half kilogram of pelleted feed daily; horses in another consumed 2 milligrams (mg) of zearalenone in addition to the pelleted feed (the "low-dose" group); and the remaining horses consumed 8 mg of zearalenone daily in addition to the pellets (the "high-dose" group).

The mares began treatment on a day they ovulated, and the study continued for three subsequent estrous cycles. The researchers monitored the mares' reproductive activity throughout the study, and mares were bred during their third estrus.

Key findings included:

The average interovulatory interval (the number of days between ovulation) was 20.3 days for control mares, 21.5 days for low-dose mares, and 21.1 days for high-dose mares.

Pregnancy rates were 6/6 in control mares, 3/6 in low-dose mares, and 7/7 in high-dose mares. These pregnancy rates did not differ significantly from one another when a pairwise comparison was performed.

Mean serum estradiol and progesterone concentrations did not differ significantly between groups on the sampled days.

King and colleagues determined that these initial results indicated zearalenone did not cause any adverse reproductive effects when consumed in low, "environmentally relevant" doses. He added that he was "not really surprised at these concentrations, because it is a fairly common contaminant and reports of adverse effects in the mare are rare and not well-documented."

About the Author

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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